New Rear Derailleur

I have a Mongoose Blackcomb which is completely stock. I am considering updating my RD from a Shimano Altus to a Deore XT. Is there anything I need to know about compatibility, such as will it work with my 8speed cassette, or should it fit? And is it worth having a bike shop install the new derailleur? Thnx in advance.

It'll work fine. Are you going high normal or low normal on the XT? Install it yourself if you are capable of using tools and following instructions, it's not difficult. Go to parktool.com for instructions on installation/adjustment. Good luck.